1. Field of the Invention
This invention is related to the general field of lenses and spectacles with variable focal length. In particular, the invention provides a new method and apparatus for adaptively controlling the focal length of liquid lenses in eyeglasses by varying the curvature of each lens as a function of the desired focal distance.
2. Description of the Prior Art
A healthy young person is typically able to focus his or her eyesight from about 25 centimeters to infinity. As a result of presbyopia, which is very common with age, as well as because of disease, genetic conditions or accidents, the ability to focus one's eyes over a wide range of viewing distances often becomes greatly diminished with time. Therefore, at some point in their lives most people need to wear eyeglasses to correct their vision.
In aid to their ability to adjust their vision for different distances, people resort to the use of bifocal or even trifocal lenses, whereby optimum correction is provided for predetermined distances. Unfortunately, these lenses do not provide good correction for intermediate distances, for which people continue to have adjustment problems. Therefore, the ideal corrective lens should have adjustable refractive power to provide variable vision compensation for any distance. This invention is directed at producing this result.
Night-vision glasses, binoculars, cameras and similar devices that contain a chain of lenses may have adjustable focal length (similar to a zoom lens), often including automatic focusing, but they have only a restricted field of view. They are also heavy and cumbersome because of the multiple lenses and mechanisms required for the variable focal length feature. As a result, these mechanisms are not yet found in standard eyewear, but several patents exist covering methods and devices to achieve this goal through the use of liquid lenses. See, for example, U.S. Pat. No. 2,437,642 to Henroteau (1948), U.S. Pat. No. 2,576,581 to Edwards (1951), U.S. Pat. No. 3,161,718 to De Luca (1964), U.S. Pat. No. 3,614,215 to Mackta (1971), U.S. Pat. No. 3,738,734 to Tait et al. (1973), U.S. Pat. No. 4,174,156 to Glorieux (1979), and U.S. Pat. No. 4,466,706 to Lamothe (1984). All of these patents disclose inventions designed to produce varied focal lengths by changing the content, shape or thickness of fluid-filled lenses, thus providing variable refractive characteristics in a single lens.
In particular, U.S. Pat. No. 4,181,408 to Senders (1980) discloses a vision compensation system that includes fluid lenses that are adaptively adjustable to correct for vision at different focal lengths. The refractive power of the lenses, mounted on a standard spectacle frame, is changed by varying their curvature as a function of the amount of liquid retained in them, which is in turn controlled by a hydraulic pump system that is operable either manually or automatically. The apparatus also includes a system of sensors for detecting the relative position of a wearer's eyes in response to the focal point of vision and for recording the corresponding refraction characteristics of the lenses. In a manual mode of operation, the optimal fluid charge and hydraulic setting of each lens are correlated with a corresponding focal distance, as determined by the wearer. By recording this information for various distances in an electronic control circuit, the system is calibrated to provide automatic response in an servo mode of operation. Thus, for a given focal distance measured by the relative position of the wearer's eyes, the system automatically produces a hydraulic setting according to the prerecorded calibration. Rangefinder apparatus for directly measuring the distance between the lenses and an object being viewed is not incorporated in the invention.
Although the Senders invention is theoretically satisfactory, it requires a complicated control system for its implementation. As described in the patent, it includes a source of infrared radiation directed at each eye and sensor arrays to receive the reflected radiation to detect the position of the junction between the sclera and the iris. Thus, by measuring the relative position of each eye, and therefore their convergence to a common point of sight, the distance of the focal point of vision can in principle be determined. In addition, the system requires special sensors to measure the quantity of liquid in the lenses, so that a correlation can be established with the eye position.
Obviously, this complex hardware is very sensitive to any change, such as in temperature, that may cause misalignment of the various components and require a new calibration. Moreover, it appears that the method used to determine the focal point of vision is restricted to the field of view directly in front of the wearer because the movement of the eyes to peripheral areas changes the relationship between the angle of reflection of the infrared beams and the vergence angle of the eyes, thus invalidating or disabling the automatic focusing capability of the apparatus.
It is apparent that the technology of providing adjustable lenses for continuous optimal compensation at all focal lengths is in its infancy and that great progress is expected to occur in the future. The present invention is directed at simplifying the methods used in the prior art for producing a control signal to the hydraulic system driving the liquid lenses and at overcoming the above described deficiencies.